The present invention relates to semi-insulating silicon carbide single crystals, and in particular, relates to methods of forming high purity semi-insulating silicon carbide single crystal wafers that are at least 100 millimeters (mm) in diameter. Because of the relationship between English and Metric units (e.g., 25.4 mm=1 inch), such wafers are also referred to as “four inch” wafers.
Silicon carbide (SiC) has a combination of electrical and physical properties that make it an attractive semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 electron-volt (eV) bandgap (6H), a 4 Megavolt per centimeter (MV/cm) electric field breakdown, a 4.9 W/cmK thermal conductivity, and a 2×10 7 centimeter per second (cm/s) electron drift velocity. Silicon carbide is also particularly useful in its ability to be made conductive by doping or semi-insulating by various processing techniques. These qualities make silicon carbide a material of choice for a vast array of electronic applications.
The production of integrated circuits for many applications, such as RF devices, requires a semi-insulating substrate on which electronic devices can be built and connected to one another. Historically, sapphire was used as substrate material for microwave devices because of its high resistance to current flow. Sapphire has the disadvantage, however, of limiting the types of semiconductor layers that may be fabricated on the substrate with appropriate crystal lattice matching for proper device operation.
As used herein, the terms “high resistivity” and “semi-insulating” can be considered synonymous for most purposes. In general, both terms describe a semiconductor material having a resistivity greater than about 1500 ohm-centimeters (ohm-cm). In this regard, semi-insulating silicon carbide devices should have a substrate resistivity of at least 1500 ohm-centimeters (ohm-cm) in order to achieve RF passive behavior. Furthermore, resistivities of 5000 ohm-cm or better are needed to minimize device transmission line losses to an acceptable level of 0.1 dB/cm or less. For device isolation and to minimize back-gating effects, the resistivity of semi-insulating silicon carbide should approach a range of 50,000 ohm-cm or higher.
Research in the field shows that the semi-insulating behavior of a silicon carbide substrate is the result of energy levels deep within the band gap of the silicon carbide; i.e., farther from both the valence band and the conduction band than the energy levels created by p-type and n-type dopants. These “deep” energy levels are believed to consist of states lying at least 300 meV away from the conduction or valence band edges, e.g., U.S. Pat. No. 5,611,955 which is representative of standard prior research in this art.
As set forth in the above-referenced patents and applications, it has been discovered that semi-insulating silicon carbide can be produced without the use of vanadium as the dopant to create deep level states that produce the semi-insulating character.
Although vanadium can produce a semi-insulating silicon carbide crystal, its presence has been observed to create a back-gating effect; i.e., the trapped negative charge on the vanadium acts as a grown-in gate in devices in which a vanadium-doped crystal is used as the semi-insulating substrate. Thus, for certain device considerations, vanadium is best avoided.
In the above-referenced issued patents, a semi-insulating silicon carbide crystal is described that includes donor dopants, acceptor dopants and intrinsic point defects that produce deep level states. When the concentration of intrinsic point defects exceeds the difference between the concentration of donors and the concentration of acceptors, the states resulting from intrinsic point defects can provide semi-insulating characteristics in the functional absence of vanadium; i.e., including a minimal presence that is less than the presence that can affect the electronic properties of the crystal.
The requirements for and the advantages of semi-insulating substrates, their use in devices, particularly microwave devices, and the associated and particular requirements for silicon carbide semi-insulating substrates are set forth in detail in the above-referenced issued patents, and are generally well understood in the art from a background standpoint. Thus, they will not be repeated in detail herein. For reference purposes, a relevant discussion is set forth in No. 6,218,680 (‘the '680 patent”) at column 1, line 14 through column 3, line 33.
To this discussion it should be added, however, that the ever-increasing demand for wireless communication services, including high bandwidth delivery of Internet access and related services, drives a corresponding demand for devices and circuits that can support such delivery, which in turn calls for materials—such as semi-insulating silicon carbide—from which devices having the required capabilities can be manufactured.
Accordingly, the '680 patent explains that superior microwave performance can be achieved by the fabrication of silicon carbide field effect transistors (FETs) and related devices on high purity, vanadium-free semi-insulating monocrystalline silicon carbide substrates. As set forth in the '680 patent, the substrates derive their semi-insulating properties from the presence of intrinsic (point defect related) deep electronic states lying near the middle of the silicon carbide bandgap. The intrinsic deep states generally arise during growth of a crystal boule at high temperatures from which substrate wafers are cut in a manner generally well understood in this art.
In devices that incorporate these substrates, and in order to provide the appropriate low-loss RF performance, the substrate must act as a low-loss dielectric medium by continuously maintaining its semi-insulating characteristics. In turn, the ability to maintain semi-insulating behavior is dependent upon the total number of intrinsic deep states in the substrate. In current practice, if the density of the intrinsic deep levels is not sufficiently high it has been observed in practice that the semi-insulating characteristics of the substrate can become reduced or functionally eliminated when subsequent steps are carried out on or using a semi-insulating silicon carbide wafer. Such steps include the growth of epitaxial layers at temperatures of about (for illustrative purposes) 1400° or above on the semi-insulating silicon carbide wafer. This in turn reduces the number of useful devices that can be formed on or incorporating the wafers.
Although the inventors do not wish to be bound by any particular theory, it appears that when semi-insulating silicon carbide substrate wafers of this type are subjected to process steps at temperatures within certain ranges, the subsequent processing can act as an anneal that reduces the number of point defects. This can be thought of in the positive sense that a higher quality crystal is created, but it is disadvantageous when the number of intrinsic point defects is the basis for the semi-insulating character of the substrate wafer.
Stated differently, if kept within a particular temperature range for a sufficient time, the crystal equilibrium or near-equilibrium will shift to one in which the number of point defects is reduced; i.e., the crystal becomes more ordered (fewer point defects) at lower temperatures than it was at higher temperatures, in a manner expected in accordance with well-understood thermodynamic principles.
As another factor, silicon carbide grown by most techniques is generally too conductive for semi-insulating purposes, however. In particular, the nominal or unintentional nitrogen concentration in silicon carbide tends to be high enough in sublimation grown crystals (≧1−2×1017 17/cm3) to provide sufficient conductivity to prevent silicon carbide from being used in devices that require a semi-insulating substrate, such as microwave devices.
A recurring issue in fabricating silicon carbide crystals for electronic devices, therefore, is the control of elemental impurities such as nitrogen within the crystal. Nitrogen content, for example, affects the color of a silicon carbide crystal. This color change can have deleterious consequences for the usefulness of a crystal in certain applications requiring luminescence, such as light emitting diodes and gemstone fabrication. The nitrogen in a crystal may also yield electrical conductivity that must be controlled for silicon carbide to have appropriate properties in diverse electronic applications. The invention herein includes a means for achieving a semi-insulating silicon carbide crystal one step of which comprises reducing the nitrogen content, and therefore the inherent conductivity of a crystal with an improved method of sublimation growth in a hydrogen ambient atmosphere.
Researchers, therefore, persistently struggle with the issue of controlling, and particularly reducing, the amount of nitrogen that is transferred from the atmosphere of a sublimation growth chamber into a growing silicon carbide crystal. Commonly assigned U.S. Pat. No. 5,718,760 to Carter et al., for example, discloses a method of reducing the nitrogen concentration in the ambient atmosphere of a silicon carbide sublimation system. The Carter '760 patent reduces the nitrogen by back filling the growth chamber with an inert gas such as argon and then evacuating the growth chamber to a very low pressure.
Another technique for decreasing the ambient nitrogen in a crystal growth system is the minimization of nitrogen content in the equipment itself. Commonly assigned U.S. Pat. No. 5,119,540 issued to Kong et al., discloses that most, if not all, of the undesired nitrogen in a crystal growth system is a result of nitrogen gas that escapes from the equipment itself. For example, nitrogen trapped in graphite equipment may leak into the ambient atmosphere because the equipment cracks or develops pin holes through which nitrogen escapes at very high temperatures. The Kong '540 patent prevents incorporation of nitrogen into subject silicon carbide crystals by utilizing fabrication equipment made of materials with low nitrogen concentration. The Kong '540 patent, therefore, teaches that extremely pure equipment components that are free of high nitrogen content result in silicon carbide crystals that are less contaminated with undesirable levels of nitrogen. Kong '540 shows nitrogen minimization in a chemical vapor deposition system but is equally pertinent in the sublimation systems discussed herein.
In addition to reducing the concentration of nitrogen, researchers also reduce the effects of unavoidable nitrogen content within a silicon carbide crystal. For example, the Carter '760 patent acknowledges that the background nitrogen in the sublimation chamber can lead to undesirable crystal color. The '760 patent, therefore, discloses a method of compensating the nitrogen content with a corresponding p-type dopant to minimize or eliminate the undesirable effects of the nitrogen. The p-type dopant and the nitrogen compensate one another and prevent undesirable color centers in the preferably colorless silicon carbide crystal of the Carter '760 invention.
The nitrogen compensation technique has also been used to prevent unintentional nitrogen doping from dominating the conductivity of silicon carbide crystals. The '680 patent discloses a further method of compensating the nitrogen content of a silicon carbide crystal grown by sublimation. Carter points out that boron may be used to compensate the inherent nitrogen. Carter '680 also utilizes the temperature gradient in the disclosed sublimation process to create point defects in a silicon carbide crystal. The Carter '680 technique pairs an undesirable nitrogen concentration in the silicon carbide crystal with a corresponding acceptor dopant, such as boron. Carter '680 then pairs any excess dopants with temperature induced point defects to yield a desired semi-insulating crystal.
Other research also concedes that unintentional nitrogen incorporation occurs in silicon carbide crystals grown by sublimation. This research tends to focus on means for minimizing the effects of the undesirable nitrogen concentration instead of preventing the nitrogen incorporation from the outset. The '955 patent cited above and issued to Barrett et al. is illustrative of this point. Barrett '955 shows a means of introducing elements such as vanadium into the semiconductor material that create deep energy states within the forbidden energy gap. The Barrett '955 method accounts for nitrogen content in a silicon carbide crystal by trapping the nitrogen and hindering electron mobility from the nitrogen. Barrett, therefore, achieves a semi-insulating silicon carbide substrate by adjusting the effects of the nitrogen instead of preventing its presence in the crystal.
The techniques set forth in the two Carter patents, which have a common assignee as the invention described and claimed herein, are useful for their respective purposes to minimize the effects of nitrogen incorporation in a silicon carbide crystal. The Barrett '955 patent requires further elemental doping and can give rise to unpredictable electrical responses in a subject silicon carbide crystal.
A need continues to exist, therefore, for a method of gaining extensive control over the incorporation of nitrogen into a silicon carbide crystal at the point of initial sublimation. By controlling the nitrogen content from the initial growth of the crystal, compensation techniques and the associated process steps may be minimized. Controlling the nitrogen incorporation also allows development of more diverse types of crystals, including crystals with varying degrees of nitrogen content for specialized purposes.
Silicon carbide is, however, a difficult material to work with because it can crystallize in over 150 polytypes, some of which are separated from one another by very small thermodynamic differences. Furthermore, because of silicon carbide's high melting point (over 2700° C. under high pressure), many processes for working silicon carbide, including epitaxial film deposition, often need to be carried out at much higher temperature than analogous reactions in other semiconductor materials.
In one sense the commercial or manufactured synthesis of silicon carbide—typically for use as an abrasive—has been carried out for well over a century, with growth of polycrystalline silicon carbide being recognized by Acheson in 1885. The genesis of growth for electronic purposes, however, was initiated much later, being generally attributed to the development of the “Lely” method (named after its inventor) in 1955. In 1978, the first seeded sublimation techniques, which are also referred to as modified Lely techniques, were carried out, and by the late 1980s, silicon carbide technology was being rapidly commercialized, primarily by the assignee of the present invention.
In a seeded sublimation technique, a seed crystal and a source powder are both placed in a reaction crucible which is heated to the sublimation temperature of the source and in a manner that produces a thermal gradient between the source and the marginally cooler seed crystal. The thermal gradient encourages vapor phase movement of materials from the source to the seed followed by condensation upon the seed and the resulting bulk crystal growth. The method is also referred to physical vapor transport (PVT).
In a typical silicon carbide growth technique, the crucible is made of graphite and is heated by induction or resistance, with the relevant coils and insulation being placed to establish and control the desired thermal gradients. The source powder is silicon carbide, as is the seed. The crucible is oriented vertically, with the source powder in the lower portions and the seed positioned at the top, typically on a seed holder; see U.S. Pat. No. 4,866,005 (reissued as No. RE34,861). These sources are exemplary, rather than limiting, descriptions of modern seeded sublimation growth techniques.
One typical and useful method for attaining high quality crystal growth is the epitaxial growth method in which a thin film of a semiconductor material is deposited upon a substrate of the same or a crystallographically similar material. In many circumstances, epitaxial growth is carried using chemical vapor deposition (“CVD”). This refers to the technique of introducing flowing source gases that contain the relevant elements into a reaction chamber in the presence of a suitable substrate under conditions of temperature, pressure and flow rate that encourage the source gases to react and form a high quality crystal of the desired material on the substrate.
For a number of reasons, the highest quality epitaxial layers (or “epilayers”) of any given semiconductor material are generally best formed upon a substrate of the same material; e.g., silicon carbide epilayers on silicon carbide substrates. Doing so helps eliminate crystal lattice mismatches or other physical incongruence such as different coefficients of thermal expansion that are almost always necessarily present when two different, even if quite similar, materials are in contact with one another.
Accordingly, attaining suitable substrate materials for both epitaxial growth and for devices is a fundamental requirement in semiconductor device manufacture. As another factor, although epitaxial growth tends to produce high quality crystals, it proceeds very slowly. As a result, using epitaxial growth techniques to obtain bulk single crystals tends to be commercially unattractive even if physically possible.
Therefore, increasing the rate at which large single crystals of silicon carbide can be grown, increasing the diameter to which they can be grown, and reducing the defect density in the large crystals remain necessary and desired goals.
Although the density of structural defects in silicon carbide bulk crystals has been continually reduced in recent years, relatively high defect concentrations still appear and have been found to be difficult to eliminate. These can cause significant problems in limiting the performance characteristics of devices made on the substrates, or in some cases can preclude useful devices altogether. For example, a typical defect density in some commercially available silicon carbide wafers can be on the order of 100 per square centimeter (cm-2). A megawatt device formed in silicon carbide, however, will require a defect-free area on the order of 0.4 cm-2. Thus, obtaining large single crystals that can be used to fabricate large surface area devices for high-voltage, high current applications remains difficult.
Although occasionally named differently, the most common defects in silicon carbide bulk crystals are generally referred to as micropipes and hexagonal voids. A micropipe is a hollow core super-screw dislocation with its Burgers vector lying along the c-axis. A number of causes have been proposed or identified for the generation of micropipes. These include excess materials such as silicon or carbon inclusions, extrinsic impurities such as metal deposits, boundary defects, and the movement or slippage of partial dislocations. See e.g., Powell et al., Growth of Low Micropipe Density SiC Wafers, Materials Science Forum, Vols. 338-340, pp 437-440 (2000).
Hexagonal voids are flat, hexagonal platelet-shaped cavities in the crystal that often have hollow tubes trailing beneath them. Some evidence shows that micropipes are associated with hexagonal voids. A relatively recent discussion of such defects (exemplary and not limiting) is set forth in Kuhr et al., Hexagonal Voids and the Formation of Micropipes During SiC Sublimation Growth, Journal of Applied Physics, Volume 89, No. 8, page 4625 (April 2001).
Recent research indicates that problems in the bulk crystals produced in a seeded sublimation technique can originate with the seed itself and the manner in which it is physically handled; e.g., Sanchez et al., Formation of Thermal Decomposition Cavities in Physical Vapor Transport of Silicon Carbide, Journal of Electronic Materials, Volume 29, No. 3, page 347 (2000). Sanchez uses the term “micropipe” to describe, “approximately cylindrical voids with diameters in the range of 0.1 μm to 5 μm that form at the core of superscrew dislocations aligned parallel or nearly parallel to the [0001] axis” Id. at 347. Sanchez refers to larger voids (“diameters from 5 μm to 100 μm”) as, “thermal decomposition cavities,” and opines that micropipes and thermal decomposition cavities arise from different causes. Id.
Based on this hypothesis and his experimental work, Sanchez suggests that migration of “silicon rich vapor” from the back (opposite to growth) surface of a seed crystal into the growth system, causes thermal decomposition cavities that generate micropipes in the seed and then in the growing crystal. Sanchez suggests, “a continuous diffusion barrier for silicon bearing species,” will reduce or eliminate such cavities. Sanchez describes a carbonized sucrose barrier for this purpose, but admits that the technique is, “not entirely reproducible.” Id. at 352.
Accordingly, in order to continue to provide improvements in the quality of single crystal silicon carbide bulk crystals, and to reduce the defect density, the source of defects at the seed must be identified and successfully addressed.